1. Field of the Invention
The present invention relates to a transistor used in a power amplifier module incorporated in a portable telephone terminal or the like.
2. Background Art
It is known that a deterioration in characteristics of a transistor is caused by an increase in temperature of the transistor during operation (see, for example, Japanese Patent Laid-Open No. 7-335673). FIG. 15 is a diagram showing the relationship between the collector current and the collector voltage of a conventional transistor. In FIG. 15, point A indicates a current with a normal bias applied. With increase in collector voltage, the current increases abruptly at point B. This is due to thermal runaway caused by the increased junction temperature of the transistor. With a further increase in the applied voltage, the transistor is broken by heat at point C.